Gasket used in a fabrication method, wherein a methylmethacrylate prepolymer or a mixture of unsaturated monomers, whose main constituent is methylmethacrylate, or the unsaturated monomer or monomers themselves is (are) introduced into a mold comprised of two parallel plates separated by said gasket, and the mold is brought to a temperature sufficient to complete or carry out the polymerization. The gasket consists of a core of plasticized poly(vinylchloride), said core being sheathed by a thin layer of thermoplastic polymers chosen from among propylene polymers and saturated polymers, said thermoplastic polymer being capable of retaining its elasticity at temperatures ranging from 20.degree. to 120.degree. C. and having a melting or softening point higher than 115.degree. C.